1. Field
This invention relates to a spin polarised magnetic device. This invention is applicable to the field of electronics, in particular for the production of memory points and MRAM (Magnetic Random Access Memory) type memory.
2. Description of Related Art
The context of the invention is that of MRAMs based on magnetic tunnel junctions (also referred to as magnetoresistive stacks) and more particularly MRAMs in which the writing operations are performed by spin transfer. FIG. 1 schematically illustrates the structure and function of such a junction bearing reference number 1. In a known manner, the magnetic tunnel junction (or magnetoresistive stack) 1 is comprised of two magnetic layers 2 and 3 separated by an oxide layer 4 forming a tunnel barrier, typically made out of aluminium or magnesium oxide. The magnetisation of one of the magnetic layers 2, referred to as a storage layer (magnetisation layer redirectable according to two substantially opposite directions) can be directed in different directions with respect to the magnetisation of the second layer 3, referred to as a reference layer, whose magnetisation is anchored in a fixed direction. This anchoring is generally performed by interaction with an adjacent antiferromagnetic layer not represented (exchange anisotropy mechanism). Different levels of resistance of the tunnel junction can be created according to the angle between the magnetisations of the two storage and reference layers. Thus, the information is stored in the magnetic element by the parallel or anti-parallel magnetic configuration of the magnetisations of the storage layer 2 and reference layer 3. The resistance variation according to the magnetic configuration is then used to read the information written in the memory cell. When the magnetisations of the magnetic layers 2 and 3 are anti-parallel, the resistance of junction 1 is increased; when the magnetisations are parallel, the resistance is reduced. The resistance variation between these two states can exceed 100% with the appropriate choice in materials. Each tunnel junction 1 constitutes a memory point.
In the most classic approach to MRAMs with writing via perpendicular magnetic fields, the tunnel junctions 1 are positioned in a square network inserted between two perpendicular networks of parallel lines: the bit lines 5 and word lines 6, with one located above and the other below the plane of the tunnel junctions 1. The junctions 1 are positioned between a transistor 7 and a bit line 5. A current passing through this bit line 5 produces a magnetic field. A current passing through the word line 6, orthogonal to the bit line 5 produces a second magnetic field. At the time of writing, transistor 7 is blocked and current pulses are sent simultaneously through the word line 6 and bit line 5, which cross each other at the level of the aforementioned memory point 1. The combination of these two fields enables the magnetisation to be switched from the storage layer of the aforementioned memory point 1 to the desired direction without affecting the magnetisation of the other memory points. In “reading” mode, transistor 7 is saturated. The current sent through the bit line 5 only crosses the memory point with an open transistor. This current enables the junction resistance to be measured. In comparison with a reference memory point, the binary state of the memory point (“0” or “1”) can thus be determined.
However, the writing mechanism described above presents some difficulties.
The first problem is a problem regarding selectivity. As the reversing process of the magnetisation of the storage layer of a junction occurs under the effect of external fields and as the reversing fields are statistically distributed, some neighbouring junctions could be accidentally reversed simply due to the effect from the magnetic field produced along the address line. For high density memories, as the size of the memory points is clearly submicronic, the number of address errors increases. This selectivity problem could be improved by introducing a writing technology referred to as “toggle” technology, described in the U.S. Pat. No. 6,545,906, however at the price of an increase in electrical consumption.
Moreover, the currents required through the bit lines and word lines to create the magnetic fields required for writing are equal to several mA (typically 5 to 7 mA). When looking to increase the density of memories or logic circuits, the cross-section of these conducting lines must be reduced whereas the current required to generate magnetic field pulses remains the same order of magnitude or even increases in magnitude. This therefore quickly approaches the electromigration limits in these lines, which are reached for typical current densities of approximately 107 A/cm2. For a current of 4 mA, this electromigration limit, for example, is reached for conductors with cross-sections substantially equal to 200 nm*200 nm.
In more recent years, other types of magnetic devices have been developed resolving the aforementioned problem. In such devices, the reversal of magnetisation is not produced by external magnetic fields but by using the action exerted by a spin polarised current that enters into the storage layer of the tunnel junction. In fact, J C. Slonczewski and L. Berger (Journ. magn. Magn. Mater. 159, L1 (1996) and Phys. Rev. B. 54, 9353 (1996)) predicted, and J. Katine et al (Phys. Rev. Lett. 84, 3149 (2000)) experimentally observed a few years later, that when a spin polarised current is injected into a magnetic nanostructure, this current exerts a torque on the nanostructure magnetisation, referred to as spin transfer torque or spin torque, that can act on the nanostructure magnetisation and in particular that can redirect it into a desired direction. This magnetic switching phenomenon by spin transfer was firstly observed in fully metallic systems, formed for example from alternating layers of Co and Cu of the type Co 20 nm/Cu 4 nm/Co 3 nm (Phys. Rev. Lett. 84, 3149 (2000)). A few years later, this same magnetic switching phenomenon by spin transfer was observed in low resistance magnetic tunnel junctions (Appl. Phys. Lett. 84, 3118 (2004)). This spin transfer phenomenon can thus be used as a new means of writing information in MRAM-type devices or logic components. J C. Slonczewski showed that this spin transfer torque has the form of a new term in the Landau Lifshitz Gilbert equation governing the magnetisation dynamic in magnetic systems. This new term of torque T exerted by the spin current on the local magnetisation is written as T=aJ M×(M×P) in which the prefactor aJ (hereinafter referred to as the spin transfer coefficient) is proportional to the current density crossing the nanostructure and to the polarisation of this current, M is the vector representing the magnetisation of the nanostructure crossed by the spin polarised current, P is the direction of polarisation of the current and the “x” sign designates the vector product. As the prefactor of the spin transfer term is proportional to the current density crossing through the nanostructure, the current density determines the magnetisation switching limit of the magnetic nanostructure and not the total current as for the approaches involving writing by magnetic field. For example, for a magnetic nanostructure with planar magnetisation of thickness d, with a small enough dimension (typically less than 100 nm) so as to be able to consider its magnetization in macrospin approximation, the nanostructure magnetisation has been shown to be capable of switching under the influence of a spin polarised current crossing through this nanostructure perpendicular to its plane, when the prefactor of the spin transfer term reaches the value of (aJ)crit=±α(2πMs+HK)+αHext≈α2πMs where α is the Gilbert damping constant, Ms is the spontaneous magnetisation of the nanostructure and HK is anisotropy field. This critical value of the prefactor aJ determines the critical value of the density of this current causing the magnetic switching, given that the two magnitudes are connected by:
      (          a      J        )    =            -                                  g                          2              ⁢                  μ        B                    M        s        2              ⁢          1      d        ⁢          J      e        ⁢    P  where g (approximately equal to 2) is the Landé factor, μB the Bohr magneton, e the electron charge, P the current polarisation and J the current density (Sun, Phys. Rev. B. 62, 570 (2000)). It should be noted that the expression of the new term of torque T=aJM×(M×P) exerted by the spin current on the local magnetisation is more particularly applicable to spin valve configurations. For a magnetic tunnel junction configuration, the new term of torque T exerted by the spin current on the local magnetisation can be written as T=aJM×(M×P)+bJM×P. This includes the first term aJM×(M×P) to which a second term bJM×P has been added, in which the prefactor bJ represents the coefficient of the so-called current induced effective field term. On the contrary to the example of a spin valve, in a magnetic tunnel junction, the term of the effective field is not negligible.
Therefore, when the lateral dimension of the memory element or logic component is reduced, the current required also reduces according to the cross-section of the element, the current being equal to the product of the current density multiplied by the cross-section of the element. Thus, this writing approach by spin transfer offers improved perspectives for developing the characteristics of the memory devices or logic devices implementing these elements in comparison to a writing method involving magnetic field pulses generated by electrical current pulses through conducting lines.
When looking to use spin transfer as a means of writing in MRAMs based on magnetic tunnel junctions, the current density at which the switching occurs is a very important factor. This current density must be low enough for the following two reasons:
The first reason relates to the possibility for this current density of passing through the tunnel barrier without causing any electrical damage. In fact, when a current density j crosses a tunnel barrier with the product RA of this resistance R multiplied by its area A, a voltage V=RA*j is created between the two junction electrodes. However, the voltage is limited by each electrical breakdown (typically to values between 0.7 V and 1.5 V according to the thickness of the tunnel barrier) and the magnetoresistance tends to rapidly decrease for RA products lower than 10 Ω.μm2. This implies that the maximum switching current density desired so as not to generate excessive electrical stresses on the tunnel barrier with a significant level of magnetoresistance must be approximately 106 A/cm2 or even 6.105 A/cm2.
The second reason is that the size of the selection transistor connected in series with the tunnel junction directly depends on the total current crossing the transistor. This current is typically equal to 700 μA per micron of width of the transistor canal. So that the size of the transistor does not restrict the minimisation of the size of the MRAM cell, the size of this transistor must be the same order of magnitude as the size of the junction. Given that the transistor width L is equal to the diameter of the tunnel junction, a writing current density Jwriting is obtained and must be such that the product JwritingL2 is approximately equal to 0.7 mA/μm*L; i.e., Jwriting must be approximately equal to 0.7 mA/μm/L. A junction size L of approximately 50 nm gives a density Jwriting of approximately 7.105 A/cm2. Therefore, the same desired order of magnitude is observed for the switching current density as is obtained for the first reason stipulated in the previous paragraph.
However, according to the current state of the art, for tunnel junctions with a MgO base and planar magnetisation, the critical current densities observed are approximately 7.106 A/cm2 for a switch lasting several nanoseconds (refer to the article by Y. Huai et al, Appl. Phys. Lett. 87, 222510 (2005)). This current density is still too high with respect to the value sought to be achieved, i.e. a value lower than 106 A/cm2.
One first known solution for lowering the critical current is described in the patent request FR2832542. This solution consists in adding a second anchored magnetisation layer to the tunnel junction on the side of the soft layer opposite the tunnel barrier, opposite to that of the first anchored layer of the tunnel junction and separated from the soft layer by a conductive spacer with an average thickness of approximately 3 to 5 nm (configuration referred to as “dual” configuration). This thickness must be large enough to enable a magnetic uncoupling process to occur between the second anchored layer and the soft layer (typically more than 2 nm). However, this must also be small enough with respect to the spin diffusion length so that the electrons keep the memory of their spin in their passage from the second anchored layer to the soft layer. In this stack, as the tunnel barrier resistance is the dominant contributor for the electrical resistance of the stack, all of the magnetoresistance of the structure originates from the tunnel junction formed by the first anchored layer/tunnel barrier/soft layer. However, from the point of view of the spin transfer, there is a cumulative effect from the two spin transfer torques exerted by each of the two anchored layers. This is stipulated in patent FR2832542 and was later demonstrated in theory by L. Berger in article Journ. Appl. Phys. 93 (2003)7693 and experimentally checked in the article by Y. Huai et al, Appl. Phys. Lett. 87, 222510 (2005). Thus, in the latter article, the addition of a second anchored magnetisation layer opposite to that of the first anchored layer of the tunnel junction has been shown to lower the critical density switching current from approximately 7.106 A/cm2 to 2.2 106 A/cm2. The critical current has been lowered by a factor of 3 due to this dual configuration; however this value still remains too high. Moreover, this configuration does not resolve the problems of the stochastic fluctuations in the reversing or switching time.
In fact, in a known manner, the anchoring direction of the reference layer or anchored layer is parallel or anti-parallel to the magnetisation of the storage layer. This configuration maximises the amplitude of the magnetoresistance. In any event, such a configuration requires the polarisation of the current exerting the spin transfer on the magnetisation of the soft layer to be initially parallel or anti-parallel to the magnetisation of the latter. However, as the spin transfer torque varies with the sine of the angle between the current polarisation and the magnetisation, the torque is initially zero with the effect that the switching is difficult to start. We must wait therefore until, under the effect of a random thermal fluctuation, a small angle appears between the magnetisation of the soft layer and the polarisation direction of the current so that the spin transfer torque increases and activates the magnetisation reversal. It thus follows that there is a waiting time between the start of the current pulse and the magnetisation reversal varying from several 100 ps to several 10 ns. This has been shown experimentally in the publication by T. Devolder et al (Phys. Rev. Lett. 100, 057206 (2008)). In one memory or logic component mode of operation, this waiting time is particularly disadvantageous as it restricts the operating speed of the memory or component.
A second solution to lower the critical switching current density is also described in the patent request FR2832542. This solution consists in using magnetisation material perpendicular to the plane of the layers. In fact, when a planar polarisation current in injected into a planar magnetisation nanostructure, the critical current density is expressed by
      J          WRin      -      plane        =            (                        2          ⁢          e                ℏ            )        ⁢                  α        ⁢                                  ⁢                  t          F                    P        ⁢          (                                                  μ              0                        ⁢                          M              S              2                                2                +                  2          ⁢          K                    )      where e is the electron charge, h is the Planck constant, tF is the thickness of the nanostructure, α is the Gilbert damping constant, P is the current polarisation, J is the current density, μ0 is the vacuum permeability, Ms is the magnetisation of the nanostructure and K is its anisotropy (Sun, Phys. Rev. B. 62, 570 (2000)). In this expression, the term
                    μ        0            ⁢              M        S        2              2    .represents the energy of the demagnetising field, which is generally much higher (typically by 1 to 2 order of magnitudes) than the anisotropy energy of the nanostructure. This term is associated with the fact that, when reversing the magnetisation, the latter must precess out of the plane of the layer, the cost of which is this energy from the demagnetising field. This leads to the aforementioned current densities of approximately 7.106 A/cm2 for simple tunnel junctions with planar magnetisation. However, if a perpendicular magnetisation nanostructure is used, in which a perpendicular polarisation spin current is injected, the switching current density becomes
      J          WRout      -      of      -      plane        =            (                        2          ⁢          e                ℏ            )        ⁢                  2        ⁢        α        ⁢                                  ⁢                  t          F                ⁢                  K          eff                    P      in which Keff represents the effective anisotropy of the nanostructure. This effective anisotropy groups together the shape anisotropy (i.e. demagnetising field anisotropy) tending to draw the magnetisation in the plane of the layer and perpendicular anisotropy (of volume or interfacial origin), tending to draw the magnetisation out of the plane. It thus follows that this effective anisotropy Keff is generally much lower than the term
                    μ        0            ⁢              M        S        2              2    +      2    ⁢    K  governing the critical current in the planar example. Consequently, smaller critical currents are expected to be obtained in perpendicular anisotropy than for planar magnetisation materials. Recent experimental results (Yoda et al, presented orally at Intermag 2008, Madrid FA04) have shown critical currents of 3.106 A/cm' in TbCo/CoFeB2 nm/MgO/CoFeB1 nm/GdCo type perpendicular magnetisation structures. This result is encouraging, however remains too high, which appears to indicate that the α/P ratio is higher in these out-of-plane magnetisation materials than with “normal” planar magnetisation materials. Perpendicular magnetisation materials must therefore be found with a low Gilbert damping constant and a high spin polarisation and offering a large amplitude of magnetoresistance (only 10% in the aforementioned study). Moreover, this configuration does not resolve the problem of the stochastic fluctuations in the magnetisation reversing time.
Another approach suggested in patent FR2817998 consists, in order to incite the switching of a planar magnetisation magnetic layer, in injecting a spin polarised current into this layer, with a polarisation direction perpendicular to the plane of the layers. Such a magnetic device 30 is illustrated in FIG. 2. The represented device 30 comprises an antiferromagnetic layer 10, a tri-layer stack 12 consisting of two magnetic layers 121 and 123 with anti-parallel planar magnetisations, separated by a non-magnetic conducting layer 122. This stack comprises the anchored layer. The device 30 also comprises an insulating layer 14 and a free magnetic layer 16. The group comprising 12, 14 and 16 constitutes a magnetic tunnel junction 15. Device 30 is complemented with a non-magnetic conducting separating layer 18 and a magnetic polarisation layer 20 with a magnetisation perpendicular to the plane of the layer. This layer 20 can be comprised of a stack of layers, made out of for example Fe/Pt, Fe/Pd, Co/Pt, Co/Pd or Co/Au etc. or made out of their ordered alloys. The polarisation layer rests on a conductor substrate 22. This entire stack is inserted between a current feed 24 and a current switching transistor 26. For the electrons transmitted through layer 20 or reflected by the latter, the spin direction is found to be directed parallel to the magnetisation of this layer, i.e. perpendicular to the plane of the various layers of junction 15 and in particular to the plane of free layer 16. The magnetisation of this layer subjected to this out-of-plane current of polarised electrons will turn according to a large angle cone with an axis perpendicular to the plane of the layer, without being capable of aligning itself with the spin direction due to the demagnetising field tending to maintain the magnetisation within the plane of the layer. FIG. 3 symbolically shows this rotation with a positive direction of current. An Oxyz trirectangular trihedron enables the different directions to be located, the Oz axis being perpendicular to the plane of the layers. In this patent FR2817998, out-of-plane magnetisation layer 20 was presumed to influence the dynamics of the magnetisation of layer 16 in a much more significant manner than the reference layer 123. In other words, layer 20 is mainly responsible for the precession movement of the magnetization of layer 16 when a current with an intensity level higher than the critical precession current travels through the structure, whereas layer 123 only mildly disturbs this precession movement by making it asymmetrical, promoting the parallel alignment of the magnetisations of layers 123 and 16 if the current is positive and anti-parallel if the current is negative. In the example shown in FIG. 3, the current being positive, the parallel state is therefore promoted. This is represented in FIG. 4 by an alternative variation in the magnetisation My of layer 16 with shorter My magnetisation maximums (of duration t2) and longer My magnetisation minimums (of duration t1). Thus, in this patent, the parallel state of the magnetisations of layer 16 and 123 (representing for example a “0”) or the anti-parallel state of magnetisation (representing for example a “1”) can be written with either of the current directions, however the duration of the current pulse must be controlled with precision in order to stop either on a low plateau (“0”) or on a high plateau (“1”).
According to this geometry, the effect from the spin transfer causes the magnetisation to precess on a cone around an axis perpendicular to the plane of the layers. Thus, when a continuous current traverses the structure, the magnetisation of the free layer 16 precesses in a continuous manner (i.e. it turns on itself in a continuous manner with maintained oscillations of My and Mx components and thus of the resistance of the junction). It should be noted that if device 30 only comprised the perpendicular polariser 20 and the soft layer 16 of which the magnetisation is precessing, no resistance variation of the stack would be produced associated with this precession movement, as the angle between the magnetisations of the perpendicular polariser and the soft layer is constant throughout the precession movement. In order to obtain a magnetoresistance effect that can be used in an MRAM application or a radiofrequency oscillator, the anchored reference layer 123 with planar magnetisation must be added, separated from the soft layer 16 by a non-magnetic spacer. By controlling the duration of the precession movement generated by the perpendicular polariser to the nearest alternation, the precession movement can be used to switch the magnetisation of the planar magnetisation layer 16 between two opposite directions. The advantage of this approach is that the switch is in principle very fast (approximately 0.3 ns) and not very sensitive to stochastic fluctuations. In any event, the precession frequencies involved typically equal several GHz. This means that, in order to perform a precession alternation to the magnetisation of the planar layer, the duration of the current pulse traversing this layer must be controlled to a level of precision of approximately 0.1 ns. In a memory chip-type electronic device, this is very difficult to achieve as the inductive and capacitive effects produced during the distribution of current pulses within the conducting lines causes delays and enlarged pulses. These deformations of the electrical signal are detrimental for the implementation of this precessional switch in a memory device.